There are various forms of pedal cycle. One, conventional, form of pedal cycle is that which is only ever driven by a cyclist applying force to the pedals thereof, such cycles sometimes being referred to as “push bikes”. Another, more recent, form of pedal cycle is the electrically-assisted pedal cycle (EAPC) in which electrical power is used to assist or replace the efforts of the rider. Both conventional pedal cycles and EAPCs may have two, three or four wheels, and, in some, cases even more. In the present document, the term “pedal cycle” is used to include both conventional pedal cycles and EAPCs.
As mentioned, in an EAPC, electrical power is used to assist, or in some cases replace, the efforts of the rider. Accordingly, EAPCs include means for storing electrical energy, such as batteries, and an electric motor arranged to propel, either in combination with pedal input, or to replace pedal input. The batteries can usually be recharged by plugging them into a supply of electrical energy, such as an outlet from a mains supply; in some cases, also by recovering energy from motion of the cycle by way of regenerative braking, and in others by generation of electricity in a series hybrid configuration. The principle of regenerative braking will be familiar to those skilled in this field of technology.
As a result, the overall effort usually required by a cyclist to pedal an EAPC is lower than for a conventional cycle.
EAPCs can usually be placed into one of two groups. The first group is that in which the cycle can provide electrical assistance on demand, at any time, regardless of whether or not the cyclist is pedalling. Cycles in this group are sometimes referred to as “e-bikes”, and can be thought of as being generally equivalent to electric mopeds, although one that is generally easier to pedal. Cycles in the second group only provide electrical assistance when the cyclist is pedalling. These are sometimes referred to as “pedelecs”.
Currently, in most European countries, including the UK, pedelecs at least are effectively legally classified as conventional bicycles and so may be ridden without a driving licence or insurance, providing electric assistance ceases at a speed of 25 kph. There are therefore few barriers to owning and operating an EAPC.
In recent years, technical advances have been made to the electro-mechanical drive arrangements and to the associated energy storage and recovery devices used in EAPCs. These advances have resulted in EAPCs that can be operated with greater efficiency, and hence greater ease, by the cyclist.
For all the reasons given above EAPCs are becoming increasing popular, particularly in some European countries.
Since pedelecs only provide assistance when the rider is pedalling, there is a requirement to make an assessment of whether this is the case. Many pedelecs achieve this through either a torque sensor, or a movement sensor, such as a cadence sensor. A movement sensor recognises when the pedals of the pedelec are rotated and switches the motor on in response. By incorporating a minimum force level, a torque sensor installation can avoid accidental initiation of the motor due to unintended slight movement of the pedals when the bike is stationary. In those installations with only a basic movement sensor there has to be a delay between the rotation of the pedals starting and the motor being initiated. That is to say, the motor is only initiated once movement has been taking place for a predetermined period of time. As a result, there is delay before the rider is provided with any assistance when moving off. Not only does this fail to assist the rider at this time, but may also provide an uncomfortable or counter-intuitive rider experience when the motor eventually starts.
In some pedelecs the motor is either off or fully switched on (or perhaps subject to manual user control to define the level of assistance desired). That is to say, there is no relationship between the rider's pedalling and the level of assistance provided once the motor has started. However, in other examples an attempt is made to introduce such a link. For example, control may be provided to cause the motor to provide greater assistance when the rider is pedalling at greater speed. However, this link can itself be counter-intuitive since the speed or cadence of the rider's turning the pedals is not directly linked to the power output in geared bicycles. In a lower gear, a given cadence represents a lower power output than it would in a higher gear. Thus, control of the power output of the motor based on the cadence of the pedals does not provide an intuitive link between the effort exerted by the rider and the assistance provided by the motor. For example, greater assistance can be achieved by a rider by switching to a lower gear in order to increase cadence without exerting any greater effort.
Torque sensors can improve the link between rider actions and the support provided by the motor. Such sensors assess not the cadence of the pedals but the torque applied, and thus more clearly reflect the intentions and activities of the rider. This may be of benefit both at the launch procedure, where accidental initiation can be avoided by requiring a threshold torque to be applied and during general riding where the feedback between the effort exerted by the rider and the input of the motor may be improved. However, torque sensors can be difficult to implement and are significantly expensive. In addition torque sensors often limit the flexibility in terms of frame type available for the bicycle, often do not provide continuous assessment of torque levels and can add to the weight of the bicycle.
In another approach, described in international patent application WO2010/092345 there is provided a system with an input electrical machine and an output electrical machine, the input electrical machine being coupled to the pedal input and the driven cycle wheel via an epicyclic gear set and the output electrical machine being used to assist in drive of the cycle. The input electrical machine is operated as a generator to at least partly power the output electrical machine as a motor. The current in the input electrical machine is controlled to ensure the torque applied by the pedals is appropriate for a desired input power set by the user, taking account of measurements of the angle of the pedal crank arms and a detected cadence. In this way, the feedback provided by the pedals is intended to offer a consistent experience. In particular, the control of the current in the input electrical machine results in an effective variation of the transmission ratio between the pedals and the electrical machine should the cyclist exceed or undershoot the desired torque levels. As such, there is an effective automatic gear change to allow the cyclist to cope with changes in conditions.
While this approach provides some benefits in user experience, during the launch process (i.e. when the bicycle is started from stationary) there is no initial feedback from either electrical machine as no current passes when stationary. Thus, the difficulties with providing an effective launch process are shared with pedelecs adopting movement or cadence sensors explained above. While a torque sensor may assist with this, such sensors suffer from the drawbacks outlined above. Moreover, implementations of the system described in WO2010/092345 have suffered from difficulties in the pedal feedback; in particular, variations in the feedback provided during the pedal cycle can feel peculiar to riders unused to such a system.
There remains, therefore, a desire to provide an improved system and method for control of pedelec motors, particularly with reference to the pedal feedback and the launch procedure of such devices.